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RE: Data storage, data exchange, data manipulation
- From: Nicolas LEHUEN <nicolas.lehuen@u...>
- To: "'xml-dev@l...'" <xml-dev@l...>
- Date: Mon, 02 Jul 2001 18:10:06 +0200
Title:
I don't think the
conceptual model as I know it (from the French MERISE method) real insulate the
Elois against the weird, physical details of the Morlock underground. Conceptual
models are stuffed with relational paradigms. We are so used to the relational
model that we don't see it, that's all. OK, the Elois don't have to care about
indexing, integrity constraints etc., but basically both the Elois and the
Morlock have to think the same way.
There is no
conceptual model equivalent for XML, because the hierarchical model is more
simple, and that it does not cares about storage issues. In a way, we could say
that XML *is* the conceptual model, because Elois can throw XML documents at
each other without having to know how the data it contains is stored somewhere
else (in a RDBMS ? ODBMS ? XDBMS ?).
You wrote
"there is no way in XML to hide physical data model away from the conceptual
data model". Granted, when you read/write an XML document, you *do* have a
hierarchical model, so simple and crude that it seems to come from the Morlocks.
But you don't have the slightiest idea of the technologies used to
produce/consume the documents.
Regarding RDF,
I have to say that I don't understand this technology. To me, it is a way to
express a hierarchical structure using explicit XML tags (like rdf:bag,
rdf:seq etc.) whereas in any XML document the structure is implicitely
created by the data tags themselves. I think I'm missing something there, can
someone explain it to me ?
Regards,
Nicolas
> -----Original
Message-----
> From: Jeff Lowery [mailto:jlowery@s...]
> Sent: Sunday, July 01, 2001 5:46 PM
> To:
'Bullard, Claude L (Len)'; Ronald Bourret; Xml-Dev (E-mail)
> Subject:
RE: Data storage, data exchange, data manipulation
>
>
> I
think it was Tom Passin who pointed out the distinction
> between
conceptual and physical data models.
Merging back with the Fabian
Pascal thread, this is exactly the point that RDBMS fundamentalists stress to
argue why XML and other "post-relational" data models are not needed: In
principle one builds a normalized conceptual model, uses that as the user's
view of the underlying data, and relies on the principle of "data
independence" to isolate issues of indexing, cacheing, and any other nasty
stuff that implementers and DB administrators need to care about away from the
user's view of the database. One colleague uses the metaphor of the Eloi
and the Morlocks to illustrate this: DB users should be like Eloi who
live in an a happy, idealized world of conceptual models, and let the
Morlocks in the back office figure out the ugly details of how to
make this all work in practice. (We're not supposed to think about what
happens to the Eloi when the Morlocks get hungry, I guess).
One
substantive critique of XML and XML databases, which I take seriously enough
to put up with the ranting of the relational fundies now and then, is that
there is no obvious way in XML to hide the physical data model away from the
conceptual data model. In a sense, we're back to the pre-relational days
where one had to understand the physical structure of the data in order to
work with it. I hope we can define a formal data model that has the
"relational" characteristic of defining mathematical operands that are
manipulated by well-defined operators, but has the "XML" characteristic of
dealing with attributed trees (or graphs, or nested sets, or whatever...i.e.,
an "infoset algebra"). Arguably, the XQuery and perhaps the RDF people are
working toward something like this goal. I'm not sure about the Query
algebra or whether this works in the RDF model, but RDBMS users can query for
information patterns without knowing or caring what order the rows or columns
in a physical table are; XML user's currently *have* to care whether
information is modeled as an element or an attribute, and (for elements
anyway) the physical order is defined as significant.
Granted, the
RDBMS people tend to say this is impossible, and many XML people say is
un-necessary (for example, arguing that there are semantic differences between
elements and attributes, and we *should* care about the distinction). And
granted, it so long as some "Morlock" can define an XSLT transform from
various conceptual schemas (oops, sorry, schemata) to a physical schema, we
can have the Marketing and Visual Basic "Eloi" happily playing with idealized
conceptual schema that hide the ugliness of physical reality.
Nevertheless, I think we in the XML world can do better ... but to do so
requires a lot more attention to fundamentals,
IMHO. --------------InterScan_NT_MIME_Boundary--
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